Re: [PATCH v3 02/18] system/memory: Better describe @plen argument of flatview_translate()

[Thomas Huth <thuth@redhat.com>]

On 30/09/2025 10.31, Philippe Mathieu-Daudé wrote:
> Hi Thomas,
> 
> On 30/9/25 10:24, Thomas Huth wrote:
>> On 30/09/2025 10.21, Philippe Mathieu-Daudé wrote:
>>> flatview_translate()'s @plen argument is output-only and can be NULL.
>>>
>>> When Xen is enabled, only update @plen_out when non-NULL.
>>>
>>> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>> ---
>>>   include/system/memory.h | 5 +++--
>>>   system/physmem.c        | 9 +++++----
>>>   2 files changed, 8 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/system/memory.h b/include/system/memory.h
>>> index aa85fc27a10..3e5bf3ef05e 100644
>>> --- a/include/system/memory.h
>>> +++ b/include/system/memory.h
>>> @@ -2992,13 +2992,14 @@ IOMMUTLBEntry 
>>> address_space_get_iotlb_entry(AddressSpace *as, hwaddr addr,
>>>    * @addr: address within that address space
>>>    * @xlat: pointer to address within the returned memory region section's
>>>    * #MemoryRegion.
>>> - * @len: pointer to length
>>> + * @plen_out: pointer to valid read/write length of the translated address.
>>> + *            It can be @NULL when we don't care about it.
>>>    * @is_write: indicates the transfer direction
>>>    * @attrs: memory attributes
>>>    */
>>>   MemoryRegion *flatview_translate(FlatView *fv,
>>>                                    hwaddr addr, hwaddr *xlat,
>>> -                                 hwaddr *len, bool is_write,
>>> +                                 hwaddr *plen_out, bool is_write,
>>>                                    MemTxAttrs attrs);
>>>   static inline MemoryRegion *address_space_translate(AddressSpace *as,
>>> diff --git a/system/physmem.c b/system/physmem.c
>>> index 8a8be3a80e2..86422f294e2 100644
>>> --- a/system/physmem.c
>>> +++ b/system/physmem.c
>>> @@ -566,7 +566,7 @@ iotlb_fail:
>>>   /* Called from RCU critical section */
>>>   MemoryRegion *flatview_translate(FlatView *fv, hwaddr addr, hwaddr *xlat,
>>> -                                 hwaddr *plen, bool is_write,
>>> +                                 hwaddr *plen_out, bool is_write,
>>>                                    MemTxAttrs attrs)
>>>   {
>>>       MemoryRegion *mr;
>>> @@ -574,13 +574,14 @@ MemoryRegion *flatview_translate(FlatView *fv, 
>>> hwaddr addr, hwaddr *xlat,
>>>       AddressSpace *as = NULL;
>>>       /* This can be MMIO, so setup MMIO bit. */
>>> -    section = flatview_do_translate(fv, addr, xlat, plen, NULL,
>>> +    section = flatview_do_translate(fv, addr, xlat, plen_out, NULL,
>>>                                       is_write, true, &as, attrs);
>>>       mr = section.mr;
>>> -    if (xen_enabled() && memory_access_is_direct(mr, is_write, attrs)) {
>>> +    if (xen_enabled() && plen_out && memory_access_is_direct(mr, is_write,
>>> +                                                             attrs)) {
>>>           hwaddr page = ((addr & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE) - 
>>> addr;
>>> -        *plen = MIN(page, *plen);
>>> +        *plen_out = MIN(page, *plen_out);
>>>       }
>>
>> My question from the previous version is still unanswered:
>>
>> https://lore.kernel.org/qemu- 
>> devel/22ff756a-51a2-43f4-8fe1-05f17ff4a371@redhat.com/
> 
> This patches
> - checks for plen not being NULL
> - describes it as
>    "When Xen is enabled, only update @plen_out when non-NULL."
> - mention that in the updated flatview_translate() documentation
>    "It can be @NULL when we don't care about it." as documented for
>    the flatview_do_translate() callee in commit d5e5fafd11b ("exec:
>    add page_mask for flatview_do_translate")
> 
> before:
> 
>    it was not clear whether we can pass plen=NULL without having
>    to look at the code.
> 
> after:
> 
>    no change when plen is not NULL, we can pass plen=NULL safely
>    (it is documented).
> 
> I shouldn't be understanding your original question, do you mind
> rewording it?

Ah, you've updated the patch in v3 to include a check for plen_out in the 
if-statement! It was not there in v2. Ok, this should be fine now:

Reviewed-by: Thomas Huth <thuth@redhat.com>

I just re-complained since you did not respond to my mail in v2, and when I 
looked at the changelog in your v3 cover letter, you did not mention the 
modification here, so I blindly assumed that this patch was unchanged.

  Thomas